Over the past century since it was first commercially used, the Bayer process has become the process of choice for extracting aluminum hydroxides from bauxite because it is economical for bauxites containing 30 to 60% Al.sub.2 O.sub.3 as aluminum hydroxides and less than 7% SiO.sub.2 as clay (kaolin) minerals. Except for the most exiguous applications, purchasers of the recovered aluminum hydroxides have voiced continuing objections to the physical characteristics attributable to impurities, mainly organic, which inevitably were inculcated in the recovered aluminum hydroxides due to the inability to remove the organic impurities essentially completely. Chief among the objections are those of "color", typically to off-white or reddish-brown crystals of aluminum hydroxide, and to the presence of oxalate (typically an alkali metal oxalate such as sodium oxalate, because sodium hydroxide is used in the Bayer process) in the aluminum hydroxide.
The objections to color are not only cosmetic because the aluminum hydroxide crystals are often used in "white" products, or in colored products where the introduction of yet another color is undesirable, but also because of the chemical contribution of sodium oxalate towards upsetting the chemistry of a process in which the aluminum hydroxide is used. Further, calcination of the aluminum hydroxide results in smelting grade alumina having a residue of sodium oxides, adding to the level of sodium. High levels of sodium are undesirable in smelting grade alumina.
Since sodium oxalate is inevitably formed in the digestion of commercially available bauxite ore, and sodium and oxalate ions remain in the filtered Bayer process liquor from which the aluminum hydroxide is precipitated, some of the oxalate ions always remain in commercially produced aluminum hydroxide. If the concentration of sodium oxalate in the process liquor is high, it crystallizes in the crystallization zone of the Bayer process and adversely affects the precipitation of aluminum hydroxide. Sodium oxalate in the aluminum hydroxide crystals causes them to crack, creating undesirable fines. The object is to decrease the concentration of sodium oxalate in the crystals, preferably by preventing its precipitation in crystalline form, and to do so economically. It is, therefore, of great commercial significance to effect even a small decrease in the level of sodium oxalate in aluminum hydroxide.
Apart from cosmetic considerations, dissolved, microscopic and submicroscopic organic materials left in the sodium aluminate solution are incorporated into the aluminum hydroxide during its precipitation. In the past, efforts to remove these impurities included using activated carbon which effectively adsorbs them. However, the cost of recovering used active carbon for reuse makes the process uneconomical, and the cost of simply disposing of the contaminated active carbon even more so.
To this day, there is no known instance of an economical purification process for Bayer liquor being practiced commercially. The problem of doing so economically is an age-old one. All the known processes for satisfactorily purifying Bayer process liquor are relatively expensive, and the quest for an effective but economical process continues unabated. The economic problem is that purchasers balk at the inclusion of the cost of purifying Bayer process liquor by any known means in the price of "white" aluminum hydroxide. The cost of carbonizing available smelting grade alumina product to make a hybrid adsorbent (so termed because it is a combination of two known adsorbents, alumina and carbon) is relatively low. The ability to calcine contaminated (or "spent") adsorbent economically, then either to use it as smelting grade alumina or recycle the calcined product to be carbonized for reuse as hybrid adsorbent, allows the use of a large quantity of the adsorbent. Since the more adsorbent used, the more effective it is, high efficiency of the product is obtained without a corresponding increase in cost. At the same time, use of the hybrid adsorbent provides a solution to the problems associated with using a mixture of separate and distinct particles of activated alumina and active carbon, then separating the spent mixture and recovering each component in a form suitable for remaking the mixture for reuse.
The Bayer process may be summarized as follows: bauxite ore is digested with 10 to 30 wt. % aqueous NaOH solution in the range of 100.degree. to 300.degree. C. with agitation to disperse and maintain the bauxite in a suspension. After decantation and sedimentation to remove bauxite residue, the liquor produced has a specific gravity ranging from 1.1 to 1.5, preferably from 1.2 to 1.3. Crystals of aluminum hydroxide are precipitated from this liquor after it is further filtered and cooled.
To minimize the color which is imparted to these crystals, the liquor has been diluted with water to a specific gravity in the range of 1.2 to 1.25, and a carbonaceous filter aid added in an amount from 0.5 to 1 wt. % of the liquor prior to the liquor being filtered. In this process, described in U.S. Pat. No. 3,002,809 to Walker, the filtrate obtained with any good grade of filter carbon as the filter aid, is still dark amber in color and still contains organic coloring matter which must be bleached. The bleached liquor is then conventionally processed to yield high purity crystals.
The cost of using a carbon filter aid, even if it were sufficiently effective to avoid bleaching the liquor obtained, appears to have been economically unacceptable even at about the time of the '809 disclosure. The amount of liquor to be treated before the aluminum hydroxide is precipitated, and the amount of carbon required to do so, are both too large. The cost of disposing of the contaminated carbon black is too high.
The process disclosed in U.S. Pat. No. 3,457,032, to de la Breteque, sought to avoid using a filter aid by flowing the liquor over an anion exchange resin, but was not commercially viable. Still other processes suggesting the use of complexing agents to remove metallic impurities, and .alpha.-methyl cellulose to remove organic impurities fared no better in the marketplace.
A recent process disclosed in U.S. Pat. No. 3,832,442 to Emerson, to improve the color of Bayer process liquor, teaches mixing it at a temperature in the range of 40.degree. to 90.degree. C. with an active alumina having a surface area of at least 50 m.sup.2 /g to remove both metallic ion and undesired color bodies simultaneously. The active alumina used does not lose more than 30% of its weight by dissolution in the alkali aluminate liquor during contact in that temperature range. Such active alumina is characterized by: loss on ignition (at 1000.degree. C. for 1 hour) less than 25 wt. %; an average pore volume of at least 0.2 cc/g; and an alumina content of at least 50 wt. %.
In a still more recent process disclosed in U.S. Pat. No. 4,275,043 to Gnyra, there is disclosed a process particularly directed to the removal of sodium oxalate from spent Bayer process liquor from which aluminum hydroxide has already been precipitated and separated. In this process, the concentration of sodium oxalate is built up to a supersaturated level and treated with activated alumina, activated carbon or activated clay adsorbents to remove only a small portion of the humic matter present, not more than 0.5 gm/liter (as organic carbon) of humic matter. The liquor is thus destabilized and allows the sodium oxalate to be precipitated, preferably by seeding with sodium oxalate crystals, optionally in the presence of flocculating agents. Gnyra teaches that the humic matter keeps the sodium oxalate in solution, and that if the humic material is removed, precipitation of the sodium oxalate is stimulated. Typically, the quantity of sodium oxalate precipitated is several times as large as the amount of humic matter removed from the liquor. Gnyra does not teach that sodium oxalate is removed by being adsorbed on the surface of the activated alumina as did Emerson.
Since precipitating the sodium oxalate was the primary goal by decreasing the amount of humic matter, it is readily seen why only activated carbon is illustrated in the examples. Activated carbon is known to be effective to remove humic matter but is inactive for the removal of sodium oxalate. Yet, in our hybrid adsorbent, the adsorption characteristics of which, for color-forming organic impurities, are similar to those of active carbon, because of the uniform distribution of the monomolecular layer of carbon over the surfaces of the pores, the hybrid adsorbent, surprisingly, also adsorbs oxalate.
The specific carbon illustrated by Gnyra has a particle size in the range of -8 to +30 mesh (U.S. Standard), a specific surface area of about 600 m.sup.2 /g and a bulk density of about 0.35 g/cc (22 lb/ft.sup.3). Clearly, the effectiveness of the activated carbon is due to the porosity of the carbon particles. The process of our invention cannot benefit from the porosity of the layer of carbon deposited on the alumina used because the layer of carbon is essentially monomolecular.
It will be evident that the ability of activated alumina to remove organic matter inures to the benefit of carbonized alumina, but the presence of the ultrathin layer of carbon on the surface of the smelting grade alumina we use allows the carbon to function as if it was a large porous mass and provides our novel material with the unexpected property of being able to remove a far greater amount of impurities from the process liquor than one might be led to expect from knowledge of the activity attributable to a given amount of either component by itself.